I'm not sure how I can contact you 'off-site' but for the purpose of the thread, here are the basics:
1) Disable unnecessary services. Ubuntu isn't my strongpoint, but, I believe debian systems use 'rcconf' to switch off services. You can also modify the files directly in /etc/rc.d. You don't want to run anything that could leave you vulnerable, or require patching if you don't need the service so it's easier just to switch off
2) IPtables. Deny by default and open up services you need such as SSH and WWW. You should also block outbound IP traffic and whitelist those services you need. In the past I've been rooted by a PHP script with poor security which was used to upload a script which downloaded a more potent payload from a remote server. If outbound traffic was denied by default it could have saved me.
3) SSH, turn off password logins and use public/private key pairs instead
4) Install logwatch and check your system daily, this will enable you to spot if something changes
5) Remote logging, if you have the opportunity, log to a remote syslog host, this way you'll have logs if your system is compromised
6) Backups and recovery plan, whilst not part of security, it's comforting to know you can recover your system with minimum impact.
Anyone else want to contribute?